A sheet feed cassette disclosed in Patent Literature 1 includes a cassette main body and a cassette extendable section. The size of the sheet feed cassette is adjusted by sliding in and out the cassette extendable section according to the size of sheets.
The sheet feed cassette includes a lock mechanism. The lock mechanism includes an engagement hole and a lock button. The engagement hole is provided in the cassette main body. The lock button is provided in the cassette extendable section.
The lock button is in engagement with the engagement hole in a locked state. Pushing down the lock button releases the lock button from the engagement hole. Thus, the locked state of the cassette main body and the cassette extendable section is released. As a result, the cassette extendable section becomes slidable relative to the cassette main body.